This application claims the priority of German patent document DE 199 54 549.9, filed on Nov. 12, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method of operating an exhaust-emission control system with a nitrogen oxide adsorber and an assigned nitrogen-oxide loading sensor, the method providing that the nitrogen oxide adsorber is operated alternately in adsorption phases with an at least stoichiometric exhaust air ratio and in regeneration phases with an at most stoichiometric exhaust air ratio.
Operating methods for exhaust-emission control systems which have a nitrogen oxide adsorber are known in many forms, in particular for cleaning exhaust gas from motor-vehicle combustion engines operated predominantly on a lean mixture. In lean operating phases of the combustion device emitting the exhaust gas to be cleaned, such as a motor-vehicle spark-ignition engine operated predominantly on a lean mixture, the nitrogen oxide adsorber stores nitrogen oxides contained in the exhaust gas by adsorption. It is possible for the nitrogen oxides not to be reduced adequately to nitrogen, for example by a three-way catalyst, because of the oxygen excess and consequently the lack of reducing agents in the exhaust gas. The loading of the nitrogen oxide adsorber, also known as a nitrogen-oxide adsorber catalyst, with nitrogen oxides, mainly in nitrate form, increases continuously in the course of a lean operating phase. When its storage capacity is exhausted and it cannot adsorb any further nitrogen oxides, a changeover is made from the lean operation of the combustion device, which corresponds to an adsorption phase of the nitrogen oxide adsorber, briefly to a rich operating phase, in which the nitrogen oxide adsorber is fed an exhaust gas with an at most stoichiometric exhaust air ratio (generally with a substoichiometric air ratio, i.e. with a rich composition of the exhaust gas). This may take place, for example, by changing over the combustion device from the previous lean operation with an at least stoichiometric oxygen component in the fuel/air mixture to be burned to rich operation with a rich mixture; by injecting reducing agents directly into the exhaust gas upstream of the nitrogen oxide adsorber; and/or by other known methods. The rich operating phase corresponds to a regeneration phase of the nitrogen oxide adsorber, in which the nitrogen oxides temporarily stored in it are desorbed and then converted by reducing agents adequately present in the fed-in rich exhaust gas. The conversion may take place, for example, in the nitrogen-oxide adsorber body itself if a three-way catalyst is integrated therein; or in a downstream nitrogen-oxide reduction catalyst; or, for example, also by exhaust gas recirculation. The use of a three-way catalyst ensures effective nitrogen oxide conversion even in the stoichiometric range of the combustion device.
During the operation of such an exhaust-emission control system, it is desirable to change over between the alternating adsorption and regeneration phases of the nitrogen oxide adsorber at the most favourable possible time in each case. In general, the longest possible lean operation of the combustion device is desired for fuel consumption reasons, interrupted only from time to time by shortest possible rich operating phases for the regeneration of the nitrogen oxide adsorber fully loaded with nitrogen oxides. To find the most favourable switching times for the changes between the usually relatively long adsorption phases and the usually relatively short regeneration phases of the nitrogen oxide adsorber, the most accurate possible knowledge of the loading state of the nitrogen oxide adsorber at a given time is to be desired.
Conventionally, it is attempted in particular to determine the loading of the nitrogen oxide adsorber indirectly in the form of an estimate of the same on the basis of operating parameters of the exhaust-emission control system and the combustion device with the assistance of a mathematical modelling of the system, see for example the laid-open patent application EP 0 598 917 A1. A further indirect method of determining the loading uses the signal of a lambda probe arranged downstream of the nitrogen oxide adsorber, see for example the laid-open patent application EP 0 733 787 A2.
Alternatively, laid-open patent application DE 196 36 790 A1 proposes a direct determination of the loading of the nitrogen oxide adsorber by means of a corresponding loading sensor system, which comprises a nitrogen oxide sensor respectively upstream and downstream of the nitrogen oxide adsorber. With this loading sensor system, the increase in loading during a respective adsorption phase can be read off directly from the differential signal of the two nitrogen oxide sensors. At the end of a respective regeneration phase, an assigned loading counter is set to zero, and a changeover is then made from the following adsorption phase to the next regeneration phase if a predetermined maximum loading state is exceeded, i.e. the loading counter has exceeded a predetermined value.
A further directly measuring nitrogen-oxide loading sensor is described in German Patent Application No. 199 16 677.3, which is not a prior publication. This loading sensor makes use of the fact that the dielectric constant of the adsorber material depends in a one-to-one way on the degree of loading, so that the loading of the nitrogen oxide adsorber with adsorbed nitrogen oxides at a given time can be ascertained directly from a measurement of the dielectric constant of the adsorber material. It is of particular advantage in the case of this loading sensor that the decrease in loading of the nitrogen oxide adsorber during a respective regeneration phase can also be continuously sensed.
As is known, during the operation of the nitrogen oxide adsorber there is frequently a gradual decrease in its storage capacity, in particular due to chemical changes on account of excessive thermal effects and due to sulphur contained in the fuel, which is adsorbed in the adsorption phases in the form of sulphur compounds, in particular sulphates, in competition with the nitrogen oxides. By suitable special regeneration phases in the form of desulphating phases, this sulphur-dependent reduction in storage capacity can be at least partially reversed. The laid-open patent application EP 0 869 263 A1 discloses the carrying out of such desulphating phases. The incorporation of sulphur into the nitrogen oxide adsorber is modelled and a respective desulphating operation is introduced if a corresponding threshold value is exceeded. In addition, the incorporation of nitrogen oxides into the nitrogen oxide adsorber during a respective adsorption phase is also estimated on a model basis. The underlying exhaust-emission control system in this case comprises, inter alia, a lambda probe respectively upstream and downstream of the nitrogen oxide adsorber.
The technical problem on which the invention is based is that of providing an operating method of the type stated at the beginning for an exhaust-emission control system with a nitrogen oxide adsorber and an associated loading sensor which makes possible in particular a comparatively good control of the changes between the adsorption and regeneration phases of the nitrogen oxide adsorber in dependence on the loading state of the nitrogen oxide adsorber.
In the case of the method according to the present invention, it is specifically provided to sense the loading of the nitrogen oxide adsorber with nitrogen oxides continuously during a respective regeneration phase with a directly measuring loading sensor provided for this purpose, and to increase the air ratio of the exhaust gas fed to the nitrogen oxide adsorber in dependence on the measured loading as the measured loading decreases. As a result, the regenerating operation is adapted in an advantageous way to the current, decreasing loading of the nitrogen oxide adsorber (i.e., the proportion of reducing agents in the exhaust gas is successively reduced in the course of the regeneration phase). A breakthrough of reducing agent, i.e. excessive reducing agent remaining in the exhaust gas emerging from the nitrogen oxide adsorber, as threatens to occur particularly towards the end of the regeneration phase, can be reliably avoided in this way.
The method according to another embodiment of the present invention likewise makes use of a directly measuring loading sensor and relates specifically to the choice of the most favourable time for ending a respective regeneration phase. For this purpose, in a first variant, the decreasing nitrogen oxide loading of the nitrogen oxide adsorber is continuously measured by the loading sensor. If the measured loading falls below a predeterminable lower threshold value, this is evaluated as a criterion for ending regeneration (i.e., the regeneration phase is ended at this time unless other criteria preclude this). In a second variant, the gradient of the continuously measured, decreasing nitrogen oxide loading is ascertained during the regeneration phase. If the amount of the loading gradient thus ascertained has fallen below a predeterminable associated threshold value, this is evaluated as a criterion for ending regeneration. In a third variant, the decrease in the nitrogen oxide loading measured by the loading sensor is continuously monitored from the beginning of the regeneration phase. As soon as this decrease in loading has reached an amount exceeding a predeterminable associated threshold value, this is evaluated as a criterion for ending regeneration.
The method according to another embodiment of the present invention uses the nitrogen-oxide loading sensor specifically for the purpose of determining from time to time the current storage capacity of the nitrogen oxide adsorber. For this purpose, the nitrogen oxide adsorber is initially saturated with nitrogen oxides in an adsorption phase. Following this, it is completely regenerated, i.e. until the loading measured by the loading sensor reaches a minimum value from which it no longer markedly decreases. The difference between the maximum loading, measured in the state of saturation at the beginning of the regeneration phase, and the minimum value at the end of the regeneration phase is then evaluated as a measure of the current storage capacity of the nitrogen oxide adsorber.
The method according to another embodiment of the present invention specifically has the aim of detecting a remaining residual loading of the nitrogen oxide adsorber, with which the nitrogen oxide adsorber remains loaded even after a regeneration phase has been completed, for example by the incorporation of sulphur or by thermal ageing. A gradual increase in the residual loading means a corresponding reduction in the storage capacity of the nitrogen oxide adsorber. For this purpose, the current loading of the nitrogen oxide adsorber is sensed with the preferably directly measuring loading sensor towards the end of a respective regeneration phase, at least at the time when a predeterminable condition for ending regeneration occurs, and the minimum loading measured in such a way is evaluated as a measure of the current remaining residual loading of the nitrogen oxide adsorber.
In another development of the present invention, the storage capacity determination or the residual loading determination is used for promptly detecting that a special regenerating operation for desulphating the nitrogen oxide adsorber should be carried out. The carrying out of a desulphating operation is indicated whenever the difference between saturation loading and minimum loading, indicative of the storage capacity at a given time, falls below an associated threshold value or whenever the minimum loading, indicative of the remaining residual loading at the end of the respective regeneration, rises over time by more than a predeterminable degree.
In a further refinement of the measure of performing a desulphating operation at certain times, a sensing of the desulphating effect, and consequently regenerating effect achieved by the respective desulphating operation, is provided. In a first alternative, the difference in loading, indicative of the storage capacity, as obtained for a final storage capacity determination before a desulphating operation, is compared with the difference in loading of a storage capacity determination carried out for the first time after the desulphating operation, and the result of the comparison (i.e., the difference between the two loading differential values) is evaluated as a measure of the desulphating effect achieved. In a second alternative, the minimum loading measured last before the desulphating process is compared with the minimum loading measured for the first time after the desulphating process, and the result of the comparison is in turn used as a measure of the desulphating effect achieved.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.